Method for producing silver nano-particles and silver nano-particles

ABSTRACT

The present invention provides a silver nano-particle production method which is safe and simple also in terms of scaled-up industrial-level production, in a so-called thermal decomposition method in which a silver-amine complex compound is thermally decomposed to form silver nano-particles. A method for producing silver nano-particles comprising: mixing an aliphatic hydrocarbon amine and a silver compound in the presence of an alcohol solvent having 3 or more carbon atoms to form a complex compound comprising the silver compound and the amine; and thermally decomposing the complex compound by heating to form silver nano-particles.

TECHNICAL FIELD

The present invention relates to a method for producing silver nano-particles and silver nano-particles. The present invention is applied also to a method for producing metal nano-particles containing a metal other than silver and metal nano-particles.

BACKGROUND ART

Silver nano-particles can be sintered even at a low temperature. Utilizing this property, a silver coating composition containing silver nano-particles is used to form electrodes or conductive circuit patterns on a substrate in production of various electronic devices. Silver nano-particles are usually dispersed in an organic solvent. Silver nano-particles have an average primary particle diameter of about several nanometers to about several tens of nanometers, and their surfaces are usually coated with an organic stabilizer (protective agent). When the substrate is a plastic film or sheet, silver nano-particles need to be sintered at a low temperature (e.g., at 200° C. or less) less than a heat resistant temperature of the plastic substrate.

Particularly, attempts have been recently made to form fine metal lines (e.g., silver lines) not only on heat-resistant polyimide substrates that are already in use as substrates for flexible printed circuit boards but also on substrates made of various plastics, such as PET (polyethylene terephthalate) and polypropylene, that have lower heat resistance than polyimide but can be easily processed and are cheap. When plastic substrates having low heat resistance are used, metal nano-particles (e.g., silver nano-particles) need to be sintered at a lower temperature.

Silver nano-particles have an average primary particle diameter of about several nanometers to about several tens of nanometers, and are more likely to agglomerate than micron (μm)-size particles. Therefore, the reduction reaction of a silver compound (thermal decomposition reaction of a silver oxalate complex compound) is performed in the presence of an organic stabilizer (protective agent such as an aliphatic amine or an aliphatic carboxylic acid) so that the surfaces of resulting silver nano-particles are coated with the organic stabilizer.

Meanwhile, silver nano-particles are used in a silver coating composition (silver ink or silver paste) in which the particles are contained in an organic solvent. In order to development conductivity, an organic stabilizer coating the silver nano-particles needs to be removed during calcining performed after application of the silver coating composition onto a substrate to sinter the silver particles. When the temperature of the calcining is low, the organic stabilizer is poorly removed. When the silver particles are not sufficiently sintered, a low resistance value cannot be achieved. That is, the organic stabilizer present on the surfaces of the silver nano-particles contributes to the stabilization of the silver nano-particles, but on the other hand, interferes with the sintering of the silver nano-particles (especially, sintering by low-temperature calcining).

The use of an aliphatic amine compound and/or an aliphatic carboxylic acid compound each having a relatively long chain (e.g., 8 or more carbon atoms) as an organic stabilizer makes it easy to stabilize silver nano-particles because it is easy to ensure space between the silver nano-particles. On the other hand, the long-chain aliphatic amine compound and/or the long-chain aliphatic carboxylic acid compound are/is poorly removed when the temperature of calcining is low.

As described above, the relationship between the stabilization of silver nano-particles and the development of a low resistance value by low-temperature calcining is a trade-off.

For example, JP-A-2008-214695 discloses a method for producing silver ultrafine particles, comprising reacting silver oxalate and oleylamine to form a complex compound containing at least silver, oleylamine and an oxalate ion; and thermally decomposing the formed complex compound to form silver ultrafine particles (claim 1). Further, JP-A-2008-214695 discloses that in the above method, a saturated aliphatic amine having 1 to 18 carbon atoms in total is reacted in addition to the silver oxalate and the oleylamine (claims 2 and 3), so that a complex compound can be easily formed, the time required to produce silver ultrafine particles can be reduced, and the silver ultrafine particles protected by these amines can be formed in higher yield (paragraph [0011]). Further, the same document discloses that a solvent such as methanol, ethanol, or water may be added when forming a complex compound (paragraph [0017]).

JP-A-2010-265543 discloses a method for producing coated silver ultrafine particles, comprising the first step of mixing a silver compound that is decomposed by heating to generate metallic silver, a mid- to short-chain alkylamine having a boiling point of 100° C. to 250° C., and a mid- to short-chain alkyldiamine having a boiling point of 100° C. to 250° C. to prepare a complex compound containing the silver compound, the alkylamine and the alkyldiamine; and the second step of thermally decomposing the complex compound (claim 3, paragraphs and [0062]). Further, the same document discloses that there are no restrictions on using a reaction solvent such as methanol, or water to form a complex compound (paragraph [0068]).

CITATION LIST Patent Documents

Patent Document 1: JP-A-2008-214695

Patent Document 2: JP-A-2010-265543

SUMMARY OF INVENTION Technical Problem

JP-A-2008-214695 discloses that a solvent such as methanol, ethanol, or water may be added to form a complex compound. However, the use of methanol or ethanol cannot increase the temperature of a reaction for thermally decomposing the complex compound, and therefore the complex compound is not sufficiently thermally decomposed, which has an adverse effect on forming silver particles. The use of water causes difficulty in completely removing water after forming silver particles.

JP-A-2010-265543 discloses that there are no restrictions on using a reaction solvent such as methanol or water to form a complex compound. However, the use of methanol cannot increase the temperature of a reaction for thermally decomposing the complex compound, and therefore the complex compound is not sufficiently thermally decomposed, which has an adverse effect on forming silver particles. The use of water causes difficulty in completely removing water after forming silver particles.

On the other hand, when forming a complex compound of a silver compound such as silver oxalate and an amine is conducted in the absence of solvent, it is very difficult to perform stirring because the silver compound is in powder form. In addition, the reaction between the silver compound and the amine is accompanied by heat generation, and therefore there is a high safety risk in terms of scaled-up industrial-level production.

As described above, regarding a so-called thermal decomposition method in which a complex compound of a silver compound such as silver oxalate and an amine is thermally decomposed to form silver nano-particles, no silver nano-particle production method has been so far developed, which is safe also in terms of scaled-up industrial-level production, and which is capable of producing silver nano-particles that can be sintered at a low temperature.

It is therefore an object of the present invention to provide a silver nano-particle production method which is safe and simple also in terms of scaled-up industrial-level production, in a so-called thermal decomposition method in which a silver-amine complex compound is thermally decomposed to form silver nano-particles. It is also an object of the present invention to provide a silver nano-particle production method which is safe and simple also in terms of scaled-up industrial-level production, and which is capable of producing silver nano-particles that can be sintered at a low temperature, in a so-called thermal decomposition method in which a silver-amine complex compound is thermally decomposed to form silver nano-particles.

Solution to Problem

The present invention includes the following aspects.

(1) A method for producing silver nano-particles comprising:

mixing an aliphatic hydrocarbon amine and a silver compound in the presence of an alcohol solvent having 3 or more carbon atoms to form a complex compound comprising the silver compound and the amine; and

thermally decomposing the complex compound by heating to form silver nano-particles.

(2) The method for producing silver nano-particles according to the above (1), wherein the silver compound is silver oxalate.

(3) The method for producing silver nano-particles according to the above (1) or (2), wherein the aliphatic hydrocarbon amine comprises an aliphatic hydrocarbon monoamine (A) comprising an aliphatic hydrocarbon group and one amino group, said aliphatic hydrocarbon group having 6 or more carbon atoms in total, and

further comprises at least one of an aliphatic hydrocarbon monoamine (B) comprising an aliphatic hydrocarbon group and one amino group, said aliphatic hydrocarbon group having 5 or less carbon atoms in total; and an aliphatic hydrocarbon diamine (C) comprising an aliphatic hydrocarbon group and two amino groups, said aliphatic hydrocarbon group having 8 or less carbon atoms in total.

(4) The method for producing silver nano-particles according to the above (3), wherein the aliphatic hydrocarbon monoamine (A) is an alkylmonoamine having 6 or more and 12 or less carbon atoms.

(5) The method for producing silver nano-particles according to the above (3) or (4), wherein the aliphatic hydrocarbon monoamine (B) is an alkylmonoamine having 2 or more and 5 or less carbon atoms.

(6) The method for producing silver nano-particles according to any one of the above (3) to (5), wherein the aliphatic hydrocarbon diamine (C) is an alkylenediamine in which one of the two amino groups is a primary amino group, and the other is a tertiary amino group.

(7) The method for producing silver nano-particles according to any one of the above (1) to (6), wherein the alcohol solvent is selected from the group consisting of butanols and hexanols.

(8) The method for producing silver nano-particles according to any one of the above (1) to (7), wherein the alcohol solvent is used in an amount of 120 parts by weight or more per 100 parts by weight of the silver compound.

(9) The method for producing silver nano-particles according to any one of the above (1) to (8), wherein the aliphatic hydrocarbon amine is used in a total amount of 1 to 50 moles per 1 mole of silver atoms in the silver compound.

A molecule of silver oxalate contains two silver atoms. The method for producing silver nano-particles according to any one of the above (1) to (8), wherein when the silver compound is silver oxalate, the aliphatic hydrocarbon amine is used in a total amount of 2 to 100 moles per 1 mole of silver oxalate.

-   -   The method for producing silver nano-particles according to any         one of the above, wherein the aliphatic hydrocarbon amine         comprises the aliphatic hydrocarbon monoamine (A) and the         aliphatic hydrocarbon monoamine (B).     -   The method for producing silver nano-particles according to any         one of the above, wherein the aliphatic hydrocarbon amine         comprises the aliphatic hydrocarbon monoamine (A) and the         aliphatic hydrocarbon diamine (C).     -   The method for producing silver nano-particles according to any         one of the above, wherein the aliphatic hydrocarbon amine         comprises the aliphatic hydrocarbon monoamine (A), the aliphatic         hydrocarbon monoamine (B), and the aliphatic hydrocarbon diamine         (C).     -   The method for producing silver nano-particles according to any         one of the above, wherein in the step of forming a complex         compound comprising the silver compound and the amine, an         aliphatic carboxylic acid is also used in addition to the         aliphatic amine.     -   The method for producing silver nano-particles according to any         one of the above, wherein in the step of forming a complex         compound comprising the silver compound and the amine, an         aliphatic carboxylic acid is not used in addition to the         aliphatic amine.

(10) Silver nano-particles produced by the method according to any one of the above (1) to (9).

-   -   Coated silver nano-particles whose surfaces are coated with a         protective agent, wherein the coated silver nano-particles are         produced by the method according to any one of the above, and         the protective agent comprises the aliphatic amine used.     -   A silver coating composition comprising silver nano-particles         produced by the method according to any one of the above, and an         organic solvent. The silver coating composition may take any         form without any limitation. For example, a silver coating         composition in which the silver nano-particles are dispersed in         suspension state in the organic solvent (silver ink), or a         silver coating composition in which the silver nano-particles         are dispersed in kneaded state in the organic solvent (silver         paste).     -   A silver conductive material comprising:     -   a substrate, and     -   a silver conductive layer obtained by applying, onto the         substrate, a silver coating composition comprising silver         nano-particles produced by the method according to any one of         the above and an organic solvent, and calcining the silver         coating composition.

The silver conductive layer may be patterned.

The calcining is performed at a temperature of 200° C. or less, for example, 150° C. or less, preferably 120° C. or less, for 2 hours or less, for example, 1 hour or less, preferably 30 minutes or less, more preferably 15 minutes or less. More specifically, the calcining is performed under conditions of about 90° C. to 120° C. and about 10 minutes to 15 minutes, for example, 120° C. and 15 minutes.

-   -   A method for producing metal nano-particles comprising:     -   mixing an aliphatic hydrocarbon amine and a metal compound in         the presence of an alcohol solvent having 3 or more carbon atoms         to form a complex compound comprising the metal compound and the         amine; and     -   thermally decomposing the complex compound by heating to form         metal nano-particles.     -   The method for producing metal nano-particles, wherein the         aliphatic hydrocarbon amine comprises an aliphatic hydrocarbon         monoamine (A) comprising an aliphatic hydrocarbon group and one         amino group, said aliphatic hydrocarbon group having 6 or more         carbon atoms in total, and     -   further comprises at least one of an aliphatic hydrocarbon         monoamine (B) comprising an aliphatic hydrocarbon group and one         amino group, said aliphatic hydrocarbon group having 5 or less         carbon atoms in total; and an aliphatic hydrocarbon diamine (C)         comprising an aliphatic hydrocarbon group and two amino groups,         said aliphatic hydrocarbon group having 8 or less carbon atoms         in total.     -   Coated metal nano-particles whose surfaces are coated with a         protective agent, wherein the coated metal nano-particles are         produced by the above method, and the protective agent comprises         the aliphatic amine used.     -   A metal coating composition comprising metal nano-particles         produced by the above method, and an organic solvent. The metal         coating composition may take any form without limitation. For         example, a metal coating composition in which the metal         nano-particles are dispersed in suspension state in the organic         solvent (metal ink), or a metal coating composition in which the         metal nano-particles are dispersed in kneaded state in the         organic solvent (metal paste).

Advantageous Effects of Invention

In the present invention, when a complex compound comprising a silver compound and an aliphatic hydrocarbon amine is formed, a silver compound in powder form and an aliphatic hydrocarbon amine are mixed in the presence of an alcohol solvent having 3 or more carbon atoms to forma complex compound comprising the silver compound and the amine. Therefore, in the step of forming a complex compound, stirring can be sufficiently performed and the heat of reaction generated by forming a complex compound can be released to the outside of a reaction system. This makes it possible to provide a silver nano-particle production method that is safe and simple also in terms of scaled-up industrial-level production.

In the present invention, when an aliphatic hydrocarbon monoamine (A) having 6 or more carbon atoms in total, and at least one of an aliphatic hydrocarbon monoamine (B) having 5 or less carbon atoms in total and an aliphatic hydrocarbon diamine (C) having 8 or less carbon atoms in total are used, as aliphatic amine compounds that function as a complex-forming agent and/or a protective agent, silver nano-particles whose surfaces are coated with these aliphatic amine compounds are formed.

The aliphatic hydrocarbon monoamine (B) and the aliphatic hydrocarbon diamine (C) each have a short carbon chain, and are therefore easily removed from the surfaces of the silver particles in a short time of 2 hours or less, for example, 1 hour or less, preferably 30 minutes or less even by low-temperature calcining at a temperature of 200° C. or less, for example, 150° C. or less, preferably 120° C. or less. In addition, the presence of the monoamine (B) and/or the diamine (C) reduces the amount of the aliphatic hydrocarbon monoamine (A) adhered to the surfaces of the silver particles. This makes it possible to easily remove these aliphatic amine compounds from the surfaces of the silver particles in such a short time as described above even by low-temperature calcining at such a low temperature as described above, thereby allowing the silver particles to be sufficiently sintered.

As described above, according to the present invention, it is possible to provide silver nano-particles that have excellent stability and can develop excellent conductivity (low resistance value) by low-temperature and short-time calcining even when having a silver film formed with a relatively large film thickness of, for example, 1 μm or more, and a method for producing such silver nano-particles. In addition, according to the present invention, it is also possible to provide a silver coating composition comprising the silver nano-particles in stable dispersion state in an organic solvent. Further, the present invention is also applied to a method for producing metal nano-particles containing a metal other than silver, and said metal nano-particles. According to the present invention, it is possible to form a conductive film or a conductive line even on any plastic substrate having low heat resistance such as a PET substrate or a polypropylene substrate.

DESCRIPTION OF EMBODIMENTS

In the present invention, silver nano-particles are produced by mixing an aliphatic hydrocarbon amine and a silver compound in the presence of an alcohol solvent having 3 or more carbon atoms to form a complex compound comprising the silver compound and the amine, and then by thermally decomposing the complex compound by heating to form silver nano particles. Therefore, a method for producing silver nano-particles according to the present invention mainly includes a complex compound-forming step, and a thermal decomposition step of the complex compound.

In this description, the term “nano-particles” means that primary particles have a size (average primary particle diameter) of less than 1,000 nm. The particle size refers to the size of a particle not including a protective agent (a stabilizer) present on (coating) the surface of the particle (i.e., refers to the size of silver itself). In the present invention, the silver nano-particles have an average primary particle diameter of, for example, 0.5 nm to 100 nm, preferably 0.5 nm to 50 nm, more preferably 0.5 nm to 25 nm, even more preferably 0.5 nm to 20 nm.

The silver compound used in the present invention is one that is easily decomposed by heating to generate metallic silver. Examples of such a silver compound that can be used include: silver carboxylates such as silver formate, silver acetate, silver oxalate, silver malonate, silver benzoate, and silver phthalate; silver halides such as silver fluoride, silver chloride, silver bromide, and silver iodide; silver sulfate, silver nitrate, silver carbonate, and the like. In terms of the fact that metallic silver is easily generated by decomposition and impurities other than silver are less likely to be generated, silver oxalate is preferably used. Silver oxalate is advantageous in that silver oxalate has a high silver content, and metallic silver is directly obtained by thermal decomposition without the need for a reducing agent, and therefore impurities derived from a reducing agent are less likely to remain.

When metal nano-particles containing another metal other than silver are produced, a metal compound that is easily decomposed by heating to generate a desired metal is used instead of the silver compound. As such a metal compound, a metal salt corresponding to the above mentioned silver compound can be used. Examples of such a metal compound include: metal carboxylates; metal halides; and metal salt compounds such as metal sulfates, metal nitrates, and metal carbonates. Among them, in terms of the fact that a metal is easily generated by decomposition and impurities other than a metal are less likely to be generated, metal oxalate is preferably used. Examples of another metal include Al, Au, Pt, Pd, Cu, Co, Cr, In, and Ni.

Further, in order to obtain a composite with silver, the above mentioned silver compound and the above mentioned compound of another metal other than silver may be used in combination. Examples of another metal include Al, Au, Pt, Pd, Cu, Co, Cr, In, and Ni. The silver composite is composed of silver and one or more other metals, and examples thereof include Au—Ag, Ag—Cu, Au—Ag—Cu, Au—Ag—Pd, and the like. The amount of silver occupies at least 20 wt %, usually at least 50 wt %, for example, at least 80 wt % of the total amount of the metals.

In the present invention, an aliphatic hydrocarbon amine and a silver compound are mixed in the presence of an alcohol solvent having 3 or more carbon atoms to form a complex compound comprising the silver compound and the amine [complex compound-forming step].

As the alcohol solvent, an alcohol having 3 to 10 carbon atoms, preferably an alcohol having 4 to 6 carbon atoms can be used. Examples of such an alcohol include n-propanol (boiling point (bp): 97° C.), isopropanol (bp: 82° C.), n-butanol (bp: 117° C.), isobutanol (bp: 107.89° C.), sec-butanol (bp: 99.5° C.), tert-butanol (bp: 82.45° C.), n-pentanol (bp: 136° C.), n-hexanol (bp: 156° C.), n-octanol (bp: 194° C.), 2-octanol (bp: 174° C.), and the like. Among them, butanols selected from n-butanol, isobutanol, sec-butanol and tert-butanol, and hexanols are preferred in consideration of the fact that the temperature of the thermal decomposition step of the complex compound subsequently performed can be increased, and post-treatment after the formation of silver nano-particles is easy. Particularly, n-butanol and n-hexanol are preferred.

In order to sufficiently stir a silver compound-alcohol slurry, the alcohol solvent is used in an amount of, for example, 120 parts by weight or more, preferably 130 parts by weight or more, more preferably 150 parts by weight or more with respect to 100 parts by weight of the silver compound. The upper limit of the amount of the alcohol-based solvent is not particularly limited, and is, for example, 1,000 parts by weight or less, preferably 800 parts by weight or less, more preferably 500 parts by weight or less with respect to 100 parts by weight of the silver compound.

In the present invention, the mixing of an aliphatic hydrocarbon amine and a silver compound in the presence of an alcohol solvent having 3 or more carbon atoms can be performed in several ways.

For example, the mixing may be performed by first mixing a solid silver compound and an alcohol solvent to obtain a silver compound-alcohol slurry [slurry-forming step], and then by adding an aliphatic hydrocarbon amine to the obtained silver compound-alcohol slurry. The slurry represents a mixture in which the solid silver compound is dispersed in the alcohol solvent. The slurry may be obtained by adding the alcohol solvent to the solid silver compound contained in a reaction container.

Alternatively, the silver compound-alcohol slurry may be added to the aliphatic hydrocarbon amine and the alcohol solvent contained in a reaction container.

In the present invention, as an aliphatic hydrocarbon amine that functions as a complex-forming agent and/or a protective agent, for example, one is used, which contains an aliphatic hydrocarbon monoamine (A) having a hydrocarbon group having 6 or more carbon atoms in total, and further contains at least one of an aliphatic hydrocarbon monoamine (B) comprising an aliphatic hydrocarbon group and one amino group, said aliphatic hydrocarbon group having 5 or less carbon atoms in total; and an aliphatic hydrocarbon diamine (C) comprising an aliphatic hydrocarbon group and two amino groups, said aliphatic hydrocarbon group having 8 or less carbon atoms in total. These respective components are usually used in the form of a liquid amine mixture, but mixing of the amines with the silver compound (or alcohol slurry thereof) does not always need to be performed using a mixture of the amines. These amines may be added one by one to the silver compound (or its alcohol slurry thereof).

Although established, the “aliphatic hydrocarbon monoamine” in this description refers to a compound composed of one to three monovalent aliphatic hydrocarbon groups and one amino group. The “hydrocarbon group” refers to a group only composed of carbon and hydrogen. However, if necessary, each of the aliphatic hydrocarbon monoamine (A) and the aliphatic hydrocarbon monoamine (B) may have, on its hydrocarbon group, a substituent group containing a hetero atom (atom other than carbon and hydrogen) such as an oxygen atom or a nitrogen atom. This nitrogen atom does not constitute an amino group.

Further, the “aliphatic hydrocarbon diamine” refers to a compound composed of a bivalent aliphatic hydrocarbon group (alkylene group), two amino groups between which said aliphatic hydrocarbon group is interposed, and, if necessary, aliphatic hydrocarbon group(s) (alkyl group(s)) substituted for hydrogen atom(s) on the amino group(s). However, if necessary, the aliphatic hydrocarbon diamine (C) may have, on its hydrocarbon′ group, a substituent group containing a hetero atom (atom other than carbon and hydrogen) such as an oxygen atom or a nitrogen atom. This nitrogen atom does not constitute an amino group.

The aliphatic hydrocarbon monoamine (A) having 6 or more carbon atoms in total has, due to its hydrocarbon chain, high performance as a protective agent (a stabilizer) onto the surfaces of resulting silver particles.

The aliphatic hydrocarbon monoamine (A) includes a primary amine, a secondary amine, and a tertiary amine. Examples of the primary amine include saturated aliphatic hydrocarbon monoamines (i.e., alkylmonoamines) such as hexylamine, heptylamine, octylamine, nonylamine, decylamine, undecylamine, dodecylamine, tridecylamine, tetradecylamine, pentadecylamine, hexadecylamine, heptadecylamine, and octadecylamine. Examples of the saturated aliphatic hydrocarbon monoamine other than the above-mentioned linear aliphatic monoamines include branched aliphatic hydrocarbon amines such as isohexylamine, 2-ethylhexylamine, and tert-octylamine. Another example of the saturated aliphatic hydrocarbon monoamine includes cyclohexylamine. Other examples of the primary amine include unsaturated aliphatic hydrocarbon monoamines (i.e., alkenylmonoamines) such as oleylamine.

Examples of the secondary amine include dialkylmonoamines such as N,N-dipropylamine, N,N-dibutylamine, N,N-dipentylamine, N,N-dihexylamine, N,N-dipeptylamine, N,N-dioctylamine, N,N-dinonylamine, N,N-didecylamine, N,N-diundecylamine, N,N-didodecylamine, N-methyl-N-propylamine, N-ethyl-N-propylamine, and N-propyl-N-butylamine. Examples of the tertiary amine include tributylamine, trihexylamine, and the like.

Among them, saturated aliphatic hydrocarbon monoamines having 6 or more carbon atoms are preferred. When the number of carbon atoms is 6 or more, space can be secured between silver particles by adsorption of amino groups to the surfaces of the silver particles, thereby improving the effect of preventing agglomeration of the silver particles. The upper limit of the number of carbon atoms is not particularly limited, but saturated aliphatic monoamines having up to 18 carbon atoms are usually preferred in consideration of ease of availability, ease of removal during calcining, etc. Particularly, alkylmonoamines having 6 to 12 carbon atoms such as hexylamine, heptylamine, octylamine, nonylamine, decylamine, undecylamine, and dodecylamine are preferably used. The above-mentioned aliphatic hydrocarbon monoamines (A) may be used singly or in combination of two or more of them.

The aliphatic hydrocarbon monoamine (B) having 5 or less carbon atoms in total has a shorter carbon chain than the aliphatic monoamine (A) having 6 or more carbon atoms in total, and therefore the function of the aliphatic hydrocarbon monoamine (B) itself as a protective agent (a stabilizer) is considered to be low. However, the aliphatic hydrocarbon monoamine (B) has a high ability to coordinate to silver in the silver compound due to its higher polarity than the aliphatic monoamine (A), and is therefore considered to have the effect of promoting complex formation. In addition, the aliphatic hydrocarbon monoamine (B) has a short carbon chain, and therefore can be removed from the surfaces of silver particles in a short time of 30 minutes or less, or 20 minutes or less, even by low-temperature calcining at a temperature of, for example, 120° C. or less, or about 100° C. or less, which is effective for low-temperature calcining of resulting silver nano-particles.

Examples of the aliphatic hydrocarbon monoamine (B) include saturated aliphatic hydrocarbon monoamines (i.e., alkylmonoamines) having 2 to 5 carbon atoms such as ethylamine, n-propylamine, isopropylamine, n-butylamine, isobutylamine, sec-butylamine, tert-butylamine, pentylamine, isopentylamine, and tert-pentylamine. Other examples of the aliphatic hydrocarbon monoamine (B) include dialkylmonoamines such as N,N-dimethylamine and N,N-diethylamine.

Among them, n-butylamine, isobutylamine, sec-butylamine, tert-butylamine, pentylamine, isopentylamine, tert-pentylamine, and the like are preferred, and the above-mentioned butylamines are particularly preferred. The above-mentioned aliphatic hydrocarbon monoamines (B) may be used singly or in combination of two or more of them.

The aliphatic hydrocarbon diamine (C) having 8 or less carbon atoms in total has a high ability to coordinate to silver in the silver compound, and therefore has the effect of promoting complex formation. Generally, aliphatic hydrocarbon diamines have higher polarity than aliphatic hydrocarbon monoamines, and therefore have a high ability to coordinate to silver in a silver compound. Further, the aliphatic hydrocarbon diamine (C) has the effect of promoting lower-temperature and shorter-time thermal decomposition in the thermal-decomposition step of the complex compound, and therefore production of silver nano-particles can be more efficiently conducted. Further, a protective film containing the aliphatic diamine (C) on silver particles has high polarity, which improves the dispersion stability, of the silver particles in a dispersion medium comprising a highly-polar solvent. Furthermore, the aliphatic diamine (C) has a short carbon chain, and therefore can be removed from the surfaces of silver particles in a short time of 30 minutes or less, or 20 minutes or less, even by low-temperature calcining at a temperature of, for example, 120° C. or less, or about 100° C. or less, which is effective for low-temperature and short-time calcining of resulting silver nano-particles.

The aliphatic hydrocarbon diamine (C) is not particularly limited, and examples thereof include ethylenediamine, N,N-dimethylethylenediamine, N,N′-dimethylethylenediamine, N,N-diethylethylenediamine, N,N′-diethylethylenediamine, 1,3-propanediamine, 2,2-dimethyl-1,3-propanediamine, N,N-dimethyl-1,3-propanediamine, N,N′-dimethyl-1,3-propanediamine, N,N-diethyl-1,3-propanediamine, N,N′-diethyl-1,3-propanediamine, 1,4-butanediamine, N,N-dimethyl-1,4-butanediamine, N,N′-dimethyl-1,4-butanediamine, N,N-diethyl-1,4-butanediamine, N,N′-diethyl-1,4-butanediamine, 1,5-pentanediamine, 1,5-diamino-2-methylpentane, 1,6-hexanediamine, N,N-dimethyl-1,6-hexanediamine, N,N′-dimethyl-1,6-hexanediamine, 1,7-heptanediamine, 1,8-octanediamine, and the like. They are all alkylenediamines having 8 or less carbon atoms in total in which at least one of the two amino groups is a primary amino group or a secondary amino group, and have a high ability to coordinate to silver in the silver compound, and therefore have the effect of promoting complex formation.

Among them, N,N-dimethylethylenediamine, N,N-diethylethylenediamine, N,N-dimethyl-1,3-propanediamine, N,N-diethyl-1,3-propanediamine, N,N-dimethyl-1,4-butanediamine, N,N-diethyl-1,4-butanediamine, N,N-dimethyl-1,6-hexanediamine, and the like are preferred, which are alkylenediamines having 8 or less carbon atoms in total in which one of the two amino groups is a primary amino group (—NH₂) and the other is a tertiary amino group (—NR¹R²). Such preferred alkylenediamines are represented by the following structural formula:

R¹R²N—R—NH₂

wherein R represents a bivalent alkylene group, R¹ and R² may be the same or different from each other and each represent an alkyl group, and the total number of carbon atoms of R, R¹, and R² is 8 or less. The alkylene group does not usually contain a hetero atom (atom other than carbon and hydrogen) such as an oxygen atom or a nitrogen atom, but if necessary, may have a substituent group containing such a hetero atom. Further, the alkyl group does not usually contain a hetero atom such as an oxygen atom or a nitrogen atom, but if necessary, may have a substituent group containing such a hetero atom.

When one of the two amino groups is a primary amino group, the ability to coordinate to silver in the silver compound is high, which is advantageous for complex formation, and when the other is a tertiary amino group, a resulting complex is prevented from having a complicated network structure because a tertiary amino group has a poor ability to coordinate to a silver atom. If a complex has a complicated network structure, there is a case where the thermal-decomposition step of the complex requires a high temperature. Among these diamines, those having 6 or less carbon atoms in total are preferred, and those having 5 or less carbon atoms in total are more preferred in terms of the fact that they can be removed from the surfaces of silver particles in a short time even by low-temperature calcining. The above-mentioned aliphatic hydrocarbon diamines (C) may be used singly or in combination of two or more of them.

The ratio between the aliphatic hydrocarbon monoamine (A) having 6 or more carbon atoms in total, and one or both of the aliphatic hydrocarbon monoamine (B) having 5 or less carbon atoms in total and the aliphatic hydrocarbon diamine (C) having 8 or less carbon atoms in total used in the present invention is not particularly limited. For example, the amount of the aliphatic monoamine (A) may be 5 mol % to 65 mol %; and

the total amount of the aliphatic monoamine (B) and the aliphatic diamine (C) may be 35 mol % to 95 mol %, on the basis of the total amount of the amines [(A)+(B)+(C)]. By setting the content of the aliphatic monoamine (A) to 5 mol % to 65 mol %, the carbon chain of the component (A), can easily fulfill its function of protecting and stabilizing the surfaces of resulting silver particles. If the content of the component (A) is less than 5 mol %, there is a case where the protective and stabilization function is poorly developed. On the other hand, if the content of the component (A) exceeds 65 mol %, the protective and stabilization function is sufficient, but the component (A) is poorly removed by low-temperature calcining.

When the aliphatic monoamine (A), and further both the aliphatic monoamine (B) and the aliphatic diamine (C) are used, the ratio among them used is not particularly limited. For example,

the amount of the aliphatic monoamine (A) may be 5 mol % to 65 mol %; the amount of the aliphatic monoamine (B) may be 5 mol % to 70 mol %; and the amount of the aliphatic diamine (C) may be 5 mol % to 50 mol %, on the basis of the total amount of the amines [(A)+(B)+(C)].

In this case, the lower limit of the content of the component (A) is preferably 10 mol % or more, more preferably 20 mol % or more. The upper limit of the content of the component (A) is preferably 65 mol % or less, more preferably 60 mol % or less.

By setting the content of the aliphatic monoamine (B) to 5 mol % to 70 mol %, the effect of promoting complex formation is easily obtained, the aliphatic monoamine (B) itself can contribute to low-temperature and short-time calcining, and the effect of facilitating the removal of the aliphatic diamine (C) from the surfaces of silver particles during calcining is easily obtained. If the content of the component (B) is less than 5 mol %, there is a case where the effect of promoting complex formation is poor, or the component (C) is poorly removed from the surfaces of silver particles during calcining. On the other hand, if the content of the component (B) exceeds 70 mol %, the effect of promoting complex formation is obtained, but the content of the aliphatic monoamine (A) is relatively reduced so that the surfaces of resulting silver particles are poorly protected and stabilized. The lower limit of the content of the component (B) is preferably 10 mol % or more, more preferably 15 mol % or more. The upper limit of the content of the component (B) is preferably 65 mol % or less, more preferably 60 mol % or less.

By setting the content of the aliphatic diamine (C) to 5 mol % to 50 mol %, the effect of promoting complex formation and the effect of promoting the thermal-decomposition of the complex are easily obtained, and further, the dispersion stability of silver particles in a dispersion medium containing a highly-polar solvent is improved because a protective film containing the aliphatic diamine (C) on silver particles has high polarity. If the content of the component (C) is less than 5 mol %, there is a case where the effect of promoting complex formation and the effect of promoting the thermal-decomposition of the complex are poor. On the other hand, if the content of the component (C) exceeds 50 mol %, the effect of promoting complex formation and the effect of promoting the thermal-decomposition of the complex are obtained, but the content of the aliphatic monoamine (A) is relatively reduced so that the surfaces of resulting silver particles are poorly protected and stabilized. The lower limit of the content of the component (C) is preferably 5 mol % or more, more preferably 10 mol % or more. The upper limit of the content of the component (C) is preferably 45 mol % or less, more preferably 40 mol % or less.

When the aliphatic monoamine (A) and the aliphatic monoamine (B) are used (without using the aliphatic diamine (C)), the ratio between them used is not particularly limited. For example, in consideration of the above-described functions of these components,

the amount of the aliphatic monoamine (A) may be 5 mol % to 65 mol %; and the amount of the aliphatic monoamine (B) may be 35 mol % to 95 mol %, on the basis of the total amount of the amines. [(A)+(B)].

When the aliphatic monoamine (A) and the aliphatic diamine (C) are used (without using the aliphatic monoamine (B)), the ratio between them used is not particularly limited. For example, in consideration of the above-described functions of these components,

the amount of the aliphatic monoamine (A) may be 5 mol % to 65 mol %; and the amount of the aliphatic diamine (C) may be 35 mol % to 95 mol %, on the basis of the total amount of the amines [(A)+(C)].

The above ratios among/between the aliphatic monoamine (A) and the aliphatic monoamine (B) and/or the aliphatic diamine (C) used are examples and may be changed in various manners.

In the present invention, the use of the aliphatic monoamine (B) and/or the aliphatic diamine (C) each having a high ability to coordinate to silver in the silver compound makes it possible, depending on their contents, to reduce the amount of the aliphatic monoamine (A) having 6 or more carbon atoms in total adhered to the surfaces of silver particles. Therefore, these aliphatic amine compounds are easily removed from the surfaces of silver particles even by the above-described low-temperature and short-time calcining so that the silver particles are sufficiently sintered.

In the present invention, the total amount of the aliphatic hydrocarbon amine [e.g., (A) and (B) and/or (C)] is not particularly limited, but may be about 1 to 50 moles per 1 mole of silver atoms in the silver compound as a starting material. If the total amount of the amine components [(A) and (B) and/or (C)] is less than 1 mole per 1 mole of the silver atoms, there is a possibility that part of the silver compound remains without being converted to a complex compound in the complex compound-forming step so that, in the subsequent thermal decomposition step, silver particles have poor uniformity and become enlarged or the silver compound remains without being thermally decomposed. On the other hand, it is considered that even when the total amount of the amine components [(A) and (B) and/or (C)] exceeds about 50 moles per 1 mole of the silver atoms, there are few advantages. By setting the total amount of the amine components to about 1 to 50 moles, the complex compound-forming step and the thermal-decomposition step of the complex compound can be successfully performed. The lower limit of the total amount of the amine components is preferably 2 mol or more, more preferably 6 mol or more per 1 mole of silver atoms in the silver compound. It is to be noted that the molecule of silver oxalate contains two silver atoms.

In the present invention, an aliphatic carboxylic acid (D) may further be used as a stabilizer to further improve the dispersibility of silver nano-particles in a dispersion medium. The aliphatic carboxylic acid (D) may be used together with the above-described amines, and may be used by adding to the liquid amine mixture. The use of the aliphatic carboxylic acid (D) may improve the stability of silver nano-particles, especially the stability of silver nano-particles in a coating material state where the silver nano-particles are dispersed in an organic solvent.

As the aliphatic carboxylic acid (D), a saturated or unsaturated aliphatic carboxylic acid is used. Examples of the aliphatic carboxylic acid include saturated aliphatic monocarboxylic acids having 4 or more carbon atoms such as butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, nonadecanoic acid, icosanoic acid, and eicosenoic acid; and unsaturated aliphatic monocarboxylic acids having 8 or more carbon atoms such as oleic acid, elaidic acid, linoleic acid, and palmitoleic acid.

Among them, saturated or unsaturated aliphatic monocarboxylic acids having 8 to 18 carbon atoms are preferred. When the number of carbon atoms is 8 or more, space can be secured between silver particles by adsorption of carboxylic groups to the surfaces of the silver particles, thereby improving the effect of preventing agglomeration of the silver particles. In consideration of ease of availability, ease of removal during calcining, etc., saturated or unsaturated aliphatic monocarboxylic compounds having up to 18 carbon atoms are usually preferred. Particularly, octanoic acid, oleic acid, and the like are preferably used. The above-mentioned aliphatic carboxylic acids (D) may be used singly or in combination of two or more of them.

When the aliphatic carboxylic acid (D) is used, the amount of the aliphatic carboxylic acid (D) used may be, for example, about 0.05 to 10 moles, preferably 0.1 to 5 moles, more preferably 0.5 to 2 moles per 1 mole of silver atoms in the silver compound as a starting material. If the amount of the component (D) is less than 0.05 moles per 1 mole of the silver atoms, the effect of improving dispersion stability obtained by adding the component (D) is poor. On the other hand, if the amount of the component (D) reaches 10 moles, the effect of improving dispersion stability is saturated and the component (D) is poorly removed by low-temperature calcining. It is to be noted that the aliphatic carboxylic, acid (D) does not necessarily need to be used.

In the present invention, an amine mixture liquid containing the aliphatic monoamine (A) and further one or both of the aliphatic monoamine (B) and the aliphatic diamine (C) is usually prepared [preparation step for amine mixture liquid].

The amine mixture liquid can be prepared by stirring the amine component (A), the amine component (B) and/or the amine component (C), and if used, the carboxylic acid component (D) in a given ratio at a room temperature.

The aliphatic hydrocarbon amine mixture liquid containing the respective amine components is added to the silver compound (or alcohol slurry thereof) to form a complex compound containing the silver compound and the amine [complex compound-forming step]. The amine components may be added to the silver compound (or alcohol slurry thereof) one by one without using a mixture liquid thereof.

When metal nano-particles containing another metal other than silver are produced, a metal compound containing a desired metal (or alcohol slurry thereof) is used instead of the silver compound (or alcohol slurry thereof).

The silver compound (or alcohol slurry thereof) or the metal compound (or alcohol slurry thereof), and a given amount of the amine mixture liquid are mixed. At this time, the mixing may be performed by stirring them at a room temperature, or may be performed by stirring them while a mixture of them is appropriately cooled to a room temperature or less because the coordination reaction of the amines to the silver compound (or the metal compound) is accompanied by heat generation. The mixing of the silver compound and the amine mixture liquid is performed in the presence of an alcohol, and therefore stirring and cooling can be successfully performed. The alcohol and excess amines function as a reaction medium.

In a conventional method for thermally decomposing a silver-amine complex, a liquid aliphatic amine component is first placed in a reaction container, and then a powder silver compound (silver oxalate) is added thereto. The liquid aliphatic amine component is flammable, and therefore addition of the powder silver compound to the liquid aliphatic amine compound is dangerous. That is, there is a risk of ignition due to static electricity generated by addition of the powder silver compound. Further, there is also a risk of a runaway exothermic reaction due to a complex-forming reaction locally caused by addition of the powder silver compound. According to the present invention, such risks can be avoided.

When a complex compound is formed, the formed complex compound generally exhibits a color corresponding to its components, and therefore the endpoint of a complex compound-forming reaction can be determined by detecting the end of a change in the color of a reaction mixture by an appropriate spectroscopic method or the like. A complex compound formed from silver oxalate is generally colorless (appears white to our eyes), but even in such a case, it is possible to determine the state of formation of a complex compound based on a change in the form of a reaction mixture such as a change in viscosity. In this way, a silver-amine complex (or a metal-amine complex) is obtained in a medium mainly containing the alcohol and the amines.

Then, the obtained complex compound is thermally decomposed by heating to form silver nano-particles [thermal-decomposition step of complex compound]. When a metal compound containing another metal other than silver is used, desired metal nano-particles are formed. The silver nano-particles (metal nano-particles) are formed without using a reducing agent. However, if necessary, an appropriate reducing agent may be used without impairing the effects of the present invention.

In such a metal-amine complex decomposition method, the amines generally play a role in controlling the mode of formation of microparticles by agglomeration of an atomic metal generated by decomposition of the metal compound, and in forming film on the surfaces of the formed metal microparticles to prevent reagglomeration of the microparticles. That is, it is considered that when the complex compound of the metal compound and the amine is heated, the metal compound is thermally decomposed to generate an atomic metal while the coordination bond of the amine to a metallic atom is maintained, and then the metallic atoms coordinated with the amine are agglomerated to form metal nano-particles coated with an amine protective film.

At this time, the thermal decomposition is preferably performed by stirring the complex compound in a reaction medium mainly containing the alcohol and the amines. The thermal decomposition may be performed in a temperature range in which coated silver nano-particles (or coated metal nano-particles) are formed, but from the viewpoint of preventing the elimination of the amine from the surfaces of silver particles (or from the surfaces of metal particles), the thermal decomposition is preferably performed at a temperature as low as possible within such a temperature range. In case of the complex compound from silver oxalate, the thermal decomposition temperature may be, for example, about 80° C. to 120° C., preferably about 95° C. to 115° C., more specifically about 100° C. to 110° C. In case of the complex compound from silver oxalate, heating at about 100° C. allows decomposition and reduction of silver ions to occur so that coated silver nano-particles can be obtained. Further, the thermal decomposition of silver oxalate itself generally occurs at about 200° C. The reason why the thermal decomposition temperature of a silver oxalate-amine complex compound is about 100° C. lower than that of silver oxalate itself is not clear, but it is estimated that a coordination polymer structure formed by pure silver oxalate is broken by forming a complex compound of silver oxalate with the amine.

Further, the thermal decomposition of the complex compound is preferably performed in an inert gas atmosphere such as argon, but may be performed in the atmosphere.

When the complex compound is thermally decomposed, a suspension exhibiting a glossy blue color is obtained. Then, the alcohol solvent, the excess amines, etc. are removed from the suspension by, for example, sedimentation of silver nano-particles (or metal nano-particles) and decantation and washing with an appropriate solvent (water or an organic solvent) to obtain desired stable coated silver nano-particles (or coated metal nano-particles) [silver nano-particle post-treatment step]. After the washing, the coated silver nano-particles are dried to obtain a powder of the desired stable coated silver nano-particles (or coated metal nano-particles).

The decantation and washing are performed using water or an organic solvent. Examples of the organic solvent that may be used include aliphatic hydrocarbon solvents such as pentane, hexane, heptane, octane, nonane, decane, undecane, dodecane, tridecane, and tetradecane; alicyclic hydrocarbon solvents such as methylcyclohexane and cyclohexane; aromatic hydrocarbon solvents such as toluene, xylene, and mesitylene; alcohol solvents such as methanol, ethanol, propanol, and butanol; acetonitrile; and mixed solvents of them.

The step of forming the silver nano-particles according to the present invention does not require the use of a reducing agent. Therefore, a by-product derived from a reducing agent is not formed, coated silver nano-particles are easily separated from a reaction system, and high-purity coated silver nano-particles are obtained. However, if necessary, an appropriate reducing agent may be used without impairing the effects of the present invention.

In this way, silver nano-particles whose surfaces are coated with a protective agent used are formed. The protective agent comprises, for example, the aliphatic monoamine (A), and further one or both of the aliphatic monoamine (B) and the aliphatic diamine (C), and further if used, the carboxylic acid (D). The ratio among/between them contained in the protective agent is the same as the ratio among/between them used in the amine mixture liquid. The same goes for the metal nano-particles.

A silver coating composition can be prepared using the obtained silver nano-particles. The silver coating composition can take any form without any limitation. For example, a silver coating composition called “silver ink” can be prepared by dispersing the silver nano-particles in suspension state in an appropriate organic solvent (dispersion medium). Alternatively, a silver coating composition called “silver paste” can be prepared by dispersing the silver nano-particles in kneaded state in an organic solvent. Examples of the organic solvent used to obtain a silver coating composition include: aliphatic hydrocarbon solvents such as pentane, hexane, heptane, octane, nonane, decane, undecane, dodecane, tridecane, and tetradecane; alicyclic hydrocarbon solvents such as cyclohexane and methylcyclohexane; aromatic hydrocarbon solvents such as toluene, xylene, and mesitylene; and alcohol solvents such as methanol, ethanol, propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, n-nonanol, and n-decanol. Examples of the organic solvent used to obtain a silver paste as a silver coating composition include terpene-based solvents such as terpineol and dihydroterpineol. The kind and amount of organic solvent used may be appropriately determined depending on a desired concentration or viscosity of the silver coating composition (silver ink, silver paste). The same goes for the metal nano-particles.

The silver nano-particle powder and the silver coating composition obtained in the present invention have excellent stability. For example, the silver nano-particle powder is stable during storage at a room temperature for 1 month or more. The silver coating composition is stable at a silver concentration of, for example, 50 wt % at a room temperature for 1 month or more without the occurrence of agglomeration and fusion.

The prepared silver coating composition is applied onto a substrate and is then calcined.

The application can be performed by a known method such as spin coating, inkjet printing, screen printing, dispenser printing, relief printing (flexography), dye sublimation printing, offset printing, laser printer printing (toner printing), intaglio printing (gravure printing), contact printing, or microcontact printing. By using such a printing technique, a patterned silver coating composition layer is obtained, and a patterned silver conductive layer is obtained by calcining.

The calcining can be performed at 200° C. or less, for example, a room temperature (25° C.) or more and 150° C. or less, preferably a room temperature (25° C.) or more and 120° C. or less. However, in order to complete the sintering of silver by short-time calcining, the calcining may be performed at a temperature of 60° C. or more and 200° C. or less, for example, 80° C. or more and 150° C. or less, preferably 90° C. or more and 120° C. or less. The time of calcining may be appropriately determined in consideration of the amount of a silver ink applied, the calcining temperature, etc., and may be, for example, several hours (e.g., 3 hours, or 2 hours) or less, preferably 1 hour or less, more preferably 30 minutes or less, even more preferably 10 minutes to 20 minutes, more specifically 10 minutes to 15 minutes.

The silver nano-particles have such a constitution as described above, and are therefore sufficiently sintered even by such low-temperature and short-time calcining. As a result, excellent conductivity (low resistance value) is developed. A silver conductive layer having a low resistance value (e.g., 15 μΩcm or less, in the range of 7 to 15 μΩcm) is formed. The resistance value of bulk silver is 1.6 μΩcm.

Since the calcining can be performed at a low temperature, not only a glass substrate or a heat-resistant plastic substrate such as a polyimide-based film but also a general-purpose plastic substrate having low heat resistance, such as a polyester-based film, e.g., a polyethylene terephthalate (PET) film and a polyethylene naphthalate (PEN) film, or a polyolefin-based film, e.g., polypropylene film, can be suitably used as a substrate. Further, short-time calcining reduces the load on such a general-purpose plastic substrate having low heat resistance, and improves production efficiency.

The silver conductive material according to the present invention can be applied to electromagnetic wave control materials, circuit boards, antennas, radiator plates, liquid crystal displays, organic EL displays, field emission displays (FEDs), IC cards, IC tags, solar cells, LED devices, organic transistors, condensers (capacitors), electronic paper, flexible batteries, flexible sensors, membrane switches, touch panels, EMI shields, and the like.

The thickness of the silver conductive layer may be appropriately determined depending on the intended use. Particularly, the use of the silver nano-particles according to the present invention makes it possible, even when a silver conductive layer having a relatively large film thickness is formed, for the silver conductive layer to have high conductivity. The thickness of the silver conductive layer may be selected from the range of, for example, 5 nm to 10 μm, preferably 100 nm to 5 μm, more preferably 300 nm to 2 μm.

The present invention has been described above with reference mainly to silver nano-particles, but is applied also to a method for producing metal nano-particles containing a metal other than silver and said metal nano-particles.

EXAMPLES

Hereinbelow, the present invention will be described more specifically with reference to examples, but is not limited to these examples.

[Specific Resistance Value of Calcined Silver Film]

The specific resistance value of an obtained calcined silver film was measured by a four-terminal method (Loresta GP MCP-T610). The measuring limit of this device is 10⁷ Ωcm.

Reagents used in Examples and Comparative Example are as follows:

n-Butylamine (MW: 73.14): reagent manufactured by Tokyo Chemical Industry Co., Ltd.; n-Hexylamine (MW: 101.19): reagent manufactured by Tokyo Chemical Industry Co., Ltd.; n-Octylamine (MW: 129.25): reagent manufactured by Tokyo Chemical Industry Co., Ltd.; Dodecylamine (MW: 185.35): reagent manufactured by Wako Pure Chemical Industries, Ltd.; N,N-Dimethyl-1,3-propanediamine (MW: 102.18): reagent manufactured by Tokyo Chemical Industry Co., Ltd.; Methanol: special grade reagent manufactured by Wako Pure Chemical Industries, Ltd.; 1-Butanol: special grade reagent manufactured by Wako Pure Chemical Industries, Ltd.; tert-Butanol: reagent manufactured by Wako Pure Chemical Industries, Ltd.; 1-Hexanol: reagent manufactured by Wako Pure Chemical Industries, Ltd.; Decalin: reagent manufactured by Tokyo Chemical Industry Co., Ltd.; Decane: reagent manufactured by Wako Pure Chemical Industries, Ltd.; and Silver oxalate (MW: 303.78): synthesized from silver nitrate (manufactured by Wako Pure Chemical Industries, Ltd.) and oxalic acid dihydrate (manufactured by Wako Pure Chemical Industries, Ltd.).

Example 1 Preparation of Silver Nano-Particles

In a 100-mL flask, 3.0 g (9.9 mmol) of silver oxalate was placed, and then 4.5 g of 1-butanol was added and the resulting mixture was stirred at a room temperature to prepare a slurry of silver oxalate in 1-butanol.

A amine mixture liquid of 8.67 g (118.5 mmol) of n-butylamine, 6.00 g (59.3 mmol) of n-hexylamine, 5.74 g (44.4 mmol) of n-octylamine, 2.75 g (14.8 mmol) of dodecylamine, and 6.05 g (59.3 mmol) of N,N-dimethyl-1,3-propanediamine was dropped into the slurry of silver oxalate in 1-butanol at 30° C. After the completion of the dropping, the resulting mixture was stirred at 30° C. for 2 hours to proceed a complex-forming reaction of a silver oxalate with amine, thereby forming a white substance (silver oxalate-amine complex).

After the silver oxalate-amine complex was formed, the reaction mixture was heated with stirring and, as a result, was brought into a reflux state at 100° C., and the temperature of the reaction mixture did not rise any further. The silver oxalate-amine complex was thermally decomposed at this reflux temperature of 100° C. to obtain a suspension in which deep blue silver nano-particles were suspended in the amine mixture liquid.

In both the complex-forming reaction step and the thermal decomposition step, stirring could be very successfully performed.

Then, the obtained suspension was cooled and 30 g of methanol was added thereto with stirring, and then the silver nano-particles were spun down by centrifugation to remove a supernatant. Then, 9 g of methanol was again added to the silver nano-particles with stirring, and then the silver nano-particles were spun down by centrifugation to remove a supernatant. In this way, wet silver nano-particles were obtained.

(Preparation and Calcining of Nano-Silver Coating Material)

Then, a mixed solvent of 1-hexanol/decalin (weight ratio=1/1) was added to the wet silver nano-particles with stirring so that a silver concentration was 40 wt % to prepare a silver nano-particle dispersion liquid. This silver nano-particle dispersion liquid was applied onto an alkali-free glass plate by spin coating to forma coating film whose film thickness after calcining was about 0.5 μm to 1.0 μm.

After being formed, the coating film was immediately calcined in a fan drying oven in a condition at 120° C. for 15 minutes to form a calcined silver film having a thickness of about 0.8 μm. The specific resistance value of the obtained calcined silver film was measured by a four-terminal method and found to be 7.0 μΩcm.

(Regarding Silver Oxalate-Amine Complex)

The white substance obtained in the process of preparing silver nano-particles was analyzed by a DSC (differential scanning calorimeter), and as a result, its average exothermic onset temperature by thermal decomposition was 102.5° C. On the other hand, silver oxalate as a starting material was also analyzed by a DSC similarly, and as a result, its average exothermic onset temperature by thermal decomposition was 218° C. That is, the white substance obtained in the process of preparing silver nano-particles had a lower thermal decomposition temperature than silver oxalate as a starting material. The results indicate that the white substance obtained in the process of preparing silver nano-particles was a material obtained by bonding between silver oxalate and the alkylamine, and the white substance was estimated to be a silver oxalate-amine complex in which the amino group of the alkylamine was coordinated to a silver atom in silver oxalate.

The DSC analysis was performed under the following conditions:

Device: DSC 6220-ASD2 (manufactured by SII Nanotechnology Inc.); Sample container: 15 μL gold-plated sealed cell (manufactured by SII Nanotechnology Inc.); Temperature rise rate: 10° C./min (room temperature to 600° C.); Atmosphere gas inside the cell: air filled at atmospheric pressure; and Atmosphere gas outside the cell: nitrogen stream (50 mL/min).

In addition, the IR spectrum of the white substance obtained in the process of preparing silver nano-particles was measured, and as a result, absorption derived from the alkyl group of the alkylamine was observed (at about 2,900 cm⁻¹ and 1,000 cm⁻¹). The result also indicates that the viscous white substance obtained in the process of preparing silver nano-particles was a material obtained by bonding between silver oxalate and the alkylamine, and the white substance was estimated to be a silver oxalate-amine complex in which an amino group was coordinated to a silver atom in silver oxalate.

Example 2

In preparation of silver nano-particles, a slurry of silver oxalate in 1-hexanol was prepared in the same manner as in Example 1 except that 4.5 g of 1-hexanol was used instead of 1-butanol for 3.0 g (9.9 mmol) of silver oxalate. Then, in the same manner as in Example 1, the amine mixture liquid was dropped into the obtained slurry of silver oxalate in 1-hexanol to form a white substance (silver oxalate-amine complex).

After the silver oxalate-amine complex was formed, the reaction mixture was heated with stirring and, as a result, was brought into a reflux state at 102° C., and the temperature of the reaction mixture did not rise any further. The silver oxalate-amine complex was thermally decomposed at this reflux temperature of 102° C. to obtain a suspension in which deep blue silver nano-particles were suspended in the amine mixture liquid. In both the complex-forming reaction step and the thermal decomposition step, stirring could be very successfully performed.

Then, in the same manner as in Example 1, a silver nano-particle dispersion liquid was prepared using the obtained suspension, and a calcined silver film having a thickness of about 0.6 μm was formed. The specific resistance value of the obtained calcined silver film was measured by a four-terminal method and found to be 15.4 μΩcm.

Example 3

In preparation of silver nano-particles, a slurry of silver oxalate in tert-butanol was prepared in the same manner as in Example 1 except that 4.5 g of tert-butanol was used instead of 1-butanol for 3.0 g (9.9 mmol) of silver oxalate. Then, in the same manner as in Example 1, the amine mixture liquid was dropped into the obtained slurry of silver oxalate in tert-butanol to form a white substance (silver oxalate-amine complex).

After the silver oxalate-amine complex was formed, the reaction mixture was heated with stirring and, as a result, was brought into a reflux state at 96° C., and the temperature of the reaction mixture did not rise any further. The silver oxalate-amine complex was thermally decomposed at this reflux temperature of 96° C. to obtain a suspension in which deep blue silver nano-particles were suspended in the amine mixture liquid. In both the complex-forming reaction step and the thermal decomposition step, stirring could be very successfully performed.

Then, in the same manner as in Example 1, a silver nano-particle dispersion liquid was prepared using the obtained suspension, and a calcined silver film having a thickness of about 0.7 μm was formed. The specific resistance value of the obtained calcined silver film was measured by a four-terminal method and found to be 12.9 μΩcm.

Comparative Example 1

In preparation of silver nano-particles, a slurry of silver oxalate in methanol was prepared in the same manner as in Example 1 except that 4.5 g of methanol was used instead of 1-butanol for 3.0 g (9.9 mmol) of silver oxalate. Then, in the same manner as in Example 1, the amine mixture liquid was dropped into the obtained slurry of silver oxalate in methanol to form a white substance (silver oxalate-amine complex).

After the silver oxalate-amine complex was formed, the reaction mixture was heated with stirring and, as a result, was brought into a reflux state at 86° C., and the temperature of the reaction mixture did not rise any further. The silver oxalate-amine complex was thermally decomposed at this reflux temperature of 86° C. to obtain a suspension in which deep blue silver nano-particles were suspended in the amine mixture liquid.

Then, the obtained suspension was treated in the same manner as in Example 1, but the silver nano-particles were not dispersed in a mixed solvent of 1-hexanol/decalin (weight ratio=1/1) (namely, the silver nano-particles did not pass through a 0.45-μm filter), and therefore a coating film could not be formed. 

1. A method for producing silver nano-particles comprising: mixing an aliphatic hydrocarbon amine and a silver compound in the presence of an alcohol solvent having 3 or more carbon atoms to form a complex compound comprising the silver compound and the amine; and thermally decomposing the complex compound by heating to form silver nano-particles.
 2. The method for producing silver nano-particles according to claim 1, wherein the silver compound is silver oxalate.
 3. The method for producing silver nano-particles according to claim 1, wherein the aliphatic hydrocarbon amine comprises an aliphatic hydrocarbon monoamine (A) comprising an aliphatic hydrocarbon group and one amino group, said aliphatic hydrocarbon group having 6 or more carbon atoms in total, and further comprises at least one of an aliphatic hydrocarbon monoamine (B) comprising an aliphatic hydrocarbon group and one amino group, said aliphatic hydrocarbon group having 5 or less carbon atoms in total; and an aliphatic hydrocarbon diamine (C) comprising an aliphatic hydrocarbon group and two amino groups, said aliphatic hydrocarbon group having 8 or less carbon atoms in total.
 4. The method for producing silver nano-particles according to claim 3, wherein the aliphatic hydrocarbon monoamine (A) is an alkylmonoamine having 6 or more and 12 or less carbon atoms.
 5. The method for producing silver nano-particles according to claim 3, wherein the aliphatic hydrocarbon monoamine (B) is an alkylmonoamine having 2 or more and 5 or less carbon atoms.
 6. The method for producing silver nano-particles according to claim 3, wherein the aliphatic hydrocarbon diamine (C) is an alkylenediamine in which one of the two amino groups is a primary amino group, and the other is a tertiary amino group.
 7. The method for producing silver nano-particles according to claim 1, wherein the alcohol solvent is selected from the group consisting of butanols and hexanols.
 8. The method for producing silver nano-particles according to claim 1, wherein the alcohol solvent is used in an amount of 120 parts by weight or more per 100 parts by weight of the silver compound.
 9. The method for producing silver nano-particles according to claim 1, wherein the aliphatic hydrocarbon amine is used in a total amount of 1 to 50 moles per 1 mole of silver atoms in the silver compound.
 10. Silver nano-particles produced by the method according to claim
 1. 